This invention relates to control arrangements for stir-friction welders, and more particularly to automatic positioning systems for stir-friction welders.
FIG. 1 is a simplified illustration of a prior-art stir-friction welding arrangement 10. In FIG. 1, an anvil 12 supports a flat workpiece 14, illustrated in phantom to make other portions of the arrangement more obvious. A friction-stir pin tool and spindle arrangement designated generally as 16 includes a spindle 18 which rotates about its axis 8, carrying with it a pin tool or ligament holding arrangement 20. The pin tool itself includes an elongated pin or ligament 22, which also rotates in consonance with the rotating spindle 18. As the spindle 18, holder 20, and pin tool 22 rotate, a relative motion is introduced between the anvil 12 (carrying the workpiece 14) and the spindle-and-pin-tool arrangement 16. The motion is represented by an arrow 24, which suggests movement of the spindle-and-pin-tool to the right in FIG. 1, assuming that the anvil 12 and workpiece 14 remain fixed. As a consequence of the friction resulting from the rotation of the pin tool 22 within the workpiece 14, that portion of the workpiece near the pin tool 22 is heated and becomes plastic. The relative motion represented by arrow 24 indicates that the portion 14a of workpiece 14, which lies to the right of pin tool 22 in FIG. 1, has not yet been welded or hot worked, while that portion 14b of workpiece 14, which lies to the left of the pin tool 22, has already been hot-worked, as suggested by the dashed hatching. Those skilled in the stir-friction hot working arts know that the described motion results in a line weld or hot working of the workpiece.
In order to make appropriate welds, the pin tool or ligament 22 of FIG. 1 must be maintained at a depth which provides full hot working of the desired region of the workpiece. The depth of plunge of the pin tool 22 is the depth to which the tip 28 of the pin tool 22 extends below the upper surface 14us, which is the surface from which the pin tool is introduced into the workpiece. The depth of plunge cannot be such that the tip 28 of pin tool 22 extends beyond the second or lower surface 141s of the workpiece, because this might weld the workpiece 14 to the anvil 12, or might damage the workpiece or pin tool. In U.S. patent application Ser. No. 09/036,915, now U.S. Pat. No. 5,971,247, the position of the pin tool is maintained at the appropriate level by a set 26 of rollers, only one of which, namely roller 26a, is illustrated. Roller 26a is affixed by a shaft 26s to the spindle or tool holder, and rotates therewith, bearing on the upper surface 14us of the workpiece 14. The position of the tip 28 of the pin tool 22 in this prior-art arrangement extends below (or beyond) the lower rolling surface of the rollers by the desired penetration of the workpiece. The penetration of the workpiece may be termed xe2x80x9caxialxe2x80x9d penetration, because the penetration is in a direction coincident with, or at least parallel to, the axis of rotation 8 of the spindle 18 and the pin tool 22.
The roller arrangement 26 for controlling the depth of penetration of the pin tool during stir-friction welding or hot working is effective, but the roller apparatus makes it inconvenient to change the penetration depth from one workpiece to another, and it is not possible to make small adjustments in the depth of penetration during a weld or hot-working procedure.
Another prior-art method which can be used to control the depth of penetration of a stir-friction pin tool into a workpiece is by the use of distance measuring devices or sensors (not illustrated) which measure the distance between the upper surface of the workpiece, corresponding to surface 14us in FIG. 1, and a reference point on the spindle holding structure. Sensors which might be used in such a prior-art arrangement include laser systems and linear variable differential transformers (LVDTs). The sensor signal can be compared with a reference value representing the desired depth of penetration, and the spindle can be moved up or down, in the absence of rollers such as 26a, in the direction of double-headed arrow 27 of FIG. 1, in response to deviation of the measured position from the calculated position. This technique provides easy change of depth of penetration, by simply adjusting the signal representing the desired penetration, but is subject to error due to the large number of dimensions which must be added and subtracted in order to arrive at the calculated value, and because of axial position variation or changes due to slack in the spindle bearings, and similar tolerances.
Improved control arrangements are desired for stir friction welding.
A method according to the invention for stir-friction welding a planar workpiece uses a rotating pin tool which includes a pin or ligament. The pin or ligament defines a diameter at locations closer to the tip of the pin tool than at a particular location along its length. The pin or ligament also includes or defines a shoulder at the location. The shoulder has a larger diameter than the pin. The method includes the steps of rotating the tool, and applying force to the pin tool with the pin tool plunged into one side of the workpiece, and with the shoulder essentially coincident with the surface of the one side of the workpiece, so that the rotating pin creates a friction-stirred region. According to an aspect of a method according to the invention, the workpiece and the rotating tool are moved laterally (in a direction orthogonal to the axis of rotation) relative to each other, so that the friction-stirred region progresses along the workpiece. During the moving step, a signal is generated which is representative of the force applied to the pin tool. A reference signal is generated which is representative of that force which is sufficient to maintain the shoulder against the one surface of the workpiece. The signal representative of the force applied to the pin tool is compared with the reference signal, for generating an error signal representative of the difference between the force applied to the pin tool and the reference signal. The error signal is used to control the step of applying force in a manner tending to maintain the shoulder in contact with, or essentially coplanar with, the one surface of the workpiece, as a result of which, or whereby, the pin maintains substantially constant plunge depth.
In a particularly advantageous mode of practicing a method according to the invention, the step of applying force includes the steps of coupling a lead screw to the pin tool and to a fixed reference point, so that rotation of the lead screw applies pressure or force to the pin tool. The shaft of a force motor is coupled to the lead screw, for rotating the lead screw in response to rotation of the force motor, as a result of which, or whereby, the force applied to the pin tool is responsive to the rotational position of the shaft of the force motor. The shaft of the force motor is rotated in response to at least the magnitude of the error signal, and preferably in opposite rotational directions in response to the variation of the error signal relative to a particular value of the error signal, which is preferably a zero value. In a most preferred embodiment of the invention, the maximum force which can be applied to the pin tool is limited.
The method according to the invention may further include initial steps which cause the pin tool to plunge into the workpiece. These steps include positioning the tip of the pin tool adjacent the one surface of the workpiece, and generating a signal representing the plunge of the pin tool relative to the one surface of the workpiece. Other steps include generating a monotonically changing signal which represents a profile of the desired depth of plunge as a function of time, generating a difference signal representing the difference between the actual plunge of the pin tool and the desired depth of plunge, rotating the pin tool, and controlling the force in response to the difference signal in such a manner that the force increases when the actual plunge is less than the desired plunge, and decreases when the actual plunge is more than the desired plunge. The step of moving the workpiece and the rotating tool laterally relative to each other begins when the actual plunge equals the desired plunge.